Dim Plage As Range, Cell As Range
Dim nbLignes as Long
nbLignes = Cells(Rows.Count, "A").End(xlUp).Row
Set Plage = Sheets("COUT PAR PÉRIODE").Range("B5:B" & nbLignes)
For Each Cell In Plage
Cell.Offset(0, 2).Value = Cell.Value / Application.WorksheetFunction.Sum(Plage)
Next Cell